1. Field of the Invention
The present invention relates to a polarization independent optical device.
2. Description of the Related Art
In optical subscriber systems, since upstream communication (optical transmission from subscribers to the central office) and downstream communication (optical transmission from the central office to the subscribers) are carried on a single optical fiber, the upstream and downstream optical signals have different wavelengths. The central office and the subscriber equipment must therefore have optical wavelength filters for the corresponding wavelengths. In general, the optical subscriber equipment, referred to as an optical network unit (ONU), includes a light emitting element and a photodetector having optical axes that must be aligned through respective wavelength filters with the optical axes of an optical splitter/combiner when the equipment is assembled.
Recently, waveguide-type optical wavelength filters, which obviate the need for optical axis alignment, have been under study. Some known optical wavelength filters of this type use Mach-Zehnder interferometers, directional couplers, and grating reflectors, as disclosed in U.S. Pat. Nos. 4,860,294, 5,764,826, 5,960,135, and 7,072,541 and Japanese Patent Application Publication No. H8-163028.
Optical wavelength filters using directional couplers exhibit a strong wavelength dependency in the passband, and are accordingly sensitive to light source wavelength error. It is also difficult to reduce the size of these optical filters, which is on the order of hundreds of micrometers.
Optical wavelength filters using gratings of high reflection efficiency can provide a fixed transmittance in the passband. To obtain high reflection efficiency, however, the grating spacing must be less than half the wavelength of the light to be transmitted, which makes it difficult to create a grating with adequate dimensional precision.
Optical wavelength filters using a Mach-Zehnder interferometer have the advantage that their wavelength characteristics can be designed on the basis of optical circuit theory.
The equivalent refractive index and coupling coefficient of an Mach-Zehnder optical wavelength filter made by using conventional submicron-order silicon wire or channel waveguides have a strong wavelength dependency, however, which creates difficulties in ONU design. This type of optical filter also exhibits extremely high polarization dependency, so it has only been possible to use one of the two (TE and TM) polarization components.